Since the advent of the easy open container, it has become well known to provide a container wall with a removable or partially removable panel portion. The panel portion is outlined or defined by a primary score line which facilitates severance from the balance of the container wall. This severance may be initiated and propagated by the use of an appliance integral with the removable panel such as a tab or pull tab. A typical type ring tab appliance is described in U.S. Pat. No. 3,349,949. In some cases the openable portion is completely severed from the container while in other cases severance is partial, i.e. only to the extent necessary to permit the desired access to the container interior.
It was frequently found desirable in score line configurations allowing severance of an entire panel or wall and in certain other score line configurations to provide a second score line of lesser depth than the primary score line. This score line, commonly referred to as an "anti-fracture score", is believed to affect the residual stresses associated with the primary score so as to prevent microcracks in or premature fracture along this primary score from container wall handling during manufacture, transport, storage, and use. This concept is more fully described in U.S. Pat. No. 3,406,866 and British Pat. No. 1,164,179. The typical spacing therein disclosed between the centerlines of the primary and anti-fracture scores is in the range of 0.050 to 0.125 inch with a spacing of 0.080 inch indicated as preferred. In container walls having the so-called "full panel" removable sectors and other configurations in which the primary score is closely adjacent to a can end chuck wall, this spacing requires that the anti-fracture score be disposed inboard of the primary score, i.e. the primary score is between the wall periphery and the anti-fracture score.
These prior art anti-fracture score configurations, however, do not provide an adequate solution for many problems posed by new developments in the packaging arts.
The development of laminated container walls having a bonded secured film of such properties as to withstand scoring and rivet forming forces without fracture or excess thinning has provided a significant advance in the art which allows the packaging of many products previously too corrosive for packaging in easy open containers. This improvement has made it desirable to exercise greater control of metal flow during scoring. Although these laminates maintain integrity in layers adhered to the sheet metal container wall, upon scoring, the laminate configuration may tend to create difficulties in metal flow during the scoring operation unless care is exercised. In particular the presence of the laminate layers appears in some manner to promote the tendency for non-uniform metal flow in a direction generally transverse to the score line during scoring. Such non-uniform metal flow may increase the susceptibility of the primary score line to microcracks or premature fracture during manufacture, transport, storage, handling or use.
Better control of metal flow during scoring is also a highly desirable objective in non-repair coating systems employed in full panel easy open ends. In the non-repair coating system the protective coating applied to the undersurface of the container wall to be scored is of such a nature that it survives the severe strain conditions encountered beneath the score line during scoring, whereas in conventional varnished coatings the coating is often fractured by the scoring operation. The non-repair systems were originally developed for beverage containers having integral opening devices. However, in full panel ends it is often desirable to have thinner score residuals than have been typical in the beverage environment in order to facilitate opening of the container. The mechanics of opening allow the beverage score line to have a thicker residual without requiring an objectionable amount of force to open. Also in some cases thicker residuals were desirable in beverage containers because the contained beverage might develop significant internal pressure, such as in the case of carbonated beverages.
However, scoring to a thinner residual may increase the probability of score line fracture or microcracking during the scoring operation. One way to minimize this tendency in conventional coating systems is to use a wide, typically about 0.004 inch, working face scoring tool. But this option is not practical in a non-repair coat system because the wider the tool face the greater the chance of damaging the non-repair coating during scoring. It is typical to use a narrow tool, about 0.0015 inch to 0.0020 inch face width, with a non-repair coating system. Thus there is the problem of providing for the thin residuals necessary for ease of opening with the narrow tool required to preserve the non-repair coating and yet maintaining score line integrity. It is believed that the prior art provides no answer to this dilemma.
The use of conventionally coated (varnished) easy open ends in mildly corrosive environments such as fish cans has posed yet another problem. In these environments corrosion occurs along the score lines of the ends causing microperforation of the end. It is believed that this corrosion is promoted by the severe grain distortion which occurs under the normal width indenter, typically about 0.004 inch wide. This grain distortion is a reflection of the metal flow caused by the indenter and will be discussed in more detail hereinafter. The apparent solution would be to score with a narrow indenter and thus minimize such grain distortion. However, as noted with reference to the non-repair coating system, the prior art does not provide any method of scoring to thin residuals with a narrow indenter.